The discovery that mitochondrial DNA can now be visualized by fluorescence light microscopy using the probe Diamidinophenylindole (DAPI) opens up the possibility of following mitochondrial configurations during a variety of cellular activities. To fully capitalize on the mtDNA configurations as guides to mitochondrial shapes, we propose to explore the large yeast species Wickerhamia fluorescens and compare it to Saccharomyces cerevisiae since the large cells allow one to resolve the appearance and dissolution of mtDNA networks, as well as mitochondria with greater ease. This will be done by first characterizing its mtDNA at the molecular level and relating this to the light and electron microscope images of the mitochondria. Studies of the relationship between these networks, mitochondria, and other cellular organelles will seek to correlate them with their biogenesis, recognizing the dual genetic contributions of nuclear and mitochondrial DNA in this regard. Experiments will be made to relate the networks to the cell cycle and the mtDNA S period. Specific labeling with thymidine which will circumvent the lack of thymidine synthetase by using uptake mutants (tup) will be done. This will make autoradiography feasible in this large cell and permit an examination of mitochondrial biogenesis on an individual cell basis. Mitochondrial forms will be examined under a variety of other conditions aimed at seeing what the effect of cellular generation times in a chemostat will have on the stage of mitochondrial morphology. The fragmentation of mitochondria that is observed in differentiated cells will also be monitored as generation times are extended to near Go conditions.